1. Field of the Invention
This invention relates to photoconductors for electrophotography, and more particularly to photoconductors having excellent physical characteristics and manifesting high sensitivity to light a long wavelength of around 800 nm or more.
2. Description of the Prior Art
An example of a conventional photoconductor will be described by referring to FIG. 1 wherein the photoconductor is prepared by forming a selenium (Se) film 12 on an electroconductive substrate 11 made of, for example, aluminum or the like with the vacuum deposition method. However, the spectral sensitivity of the Se photoconductor decreases rapidly, when the wavelength of irradiation light exceeds 500 nm as indicated by curve A in FIG. 4. In this case, the value of the sensitivity is represented by a reciprocal of an amount of light exposure required for reducing the initial potential to one half (The same definition is also used hereinafter).
As another example of conventional photoconductors, there has been proposed a Se-Te alloy photoconductor which is prepared by adding tellurium (Te) to selenium (Se) so that the spectral sensitivity will shift toward the longer wavelengths. In such an Se-Te alloy photoconductor, however, the surface charge retention characteristics becomes inferior with increase in the amount of Te added so that such photoconductor cannot actually be utilized as a photosensitive material. In order to improve the above-mentioned charge retention characteristics, there has been also used a two-layer type photoconductor, which is prepared by forming an SE layer 22 having a thickness of about 50 .mu.m on an electroconductive substrate 21 and then forming an Se-Te alloy layer 23 having a thickness of 2-3 m on the Se layer 22 as shown in FIG. 2. In such a two-layer type photoconductor, however, when the surface Se-Te alloy layer 23 consists of Se.sub.85 and Te.sub.15 the spectral sensitivity thereof decreases rapidly at a wavelength of 650 nm or more and at a wavelength of more than 700 .mu.m the sensitivity decreases further to such extent that the photoconductor of this type cannot practically be utilized as indicated by curve B in FIG. 4.
As a still further example of conventional photoconductors, there has been proposed a two-layer function separating type photoconductor shown in FIG. 3 which is prepared by forming a carrier generation layer 32 on an aluminum substrate 31 which comprise a chlorodianeblue or squarylium acid derivative coating, and then overcoating polyvinylcarbazole or a mixture of pyrazoline and polycarbonate resin to form a carrier transport layer 33. This type of photoconductor has a sufficiently high sensitivity to visible light, but it can scarcely be employed for irradiation light having a wavelength of 650 nm or more as the photosensitive material as in the foregoing conventional photoconductors.
Recently, various attempts have been made to utilize a semiconductor laser as a light source in a laser beam printer or the like wherein laser light is utilized as the light source and a photoconductor for use in electrophotography is employed for the purpose of improving the functions thereof. Such semiconductor laser light source generally has a wavelength of 800-850 nm.
A mentioned above, at present, photoconductors having high sensitivity to light of 800 nm or longer wavelength are scarcely used so that development of a photoconductor having high sensitivity to light having a wavelength of 800 nm or longer is strongly desired.